Method of driving liquid-crystal display

ABSTRACT

A liquid-crystal display is driven by pulse-width modulation. One frame of an input video signal is divided into a plurality of subframes. A first pulse signal is applied to a liquid crystal irrespective of the level of the input video signal. A pulse width of the first pulse signal corresponds to the duration of the subframe. Application of the first pulse signal only does not drive the liquid crystal. A second pulse signal is applied to the liquid crystal in accordance with the level of the input video signal so that second pulses of the second pulse signal are superimposed on the first pulses at the same polarity, to perform pulse-width modulation to the liquid crystal. An average duration P of the subframes and a response time L obtained by adding a rise time and a fall time of the liquid crystal meet the requirements P&lt;L and P≦0.15×L.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a method of driving aliquid-crystal display mounted on projection displays, view finders,head-mount displays, etc.

[0002] Active-matrix displays are usually driven by an analog signalthat controls a drive voltage for liquid crystals, such as, disclosed inJapanese Unexamined Patent Publication No. 11-174410 (1999).

[0003] There are several modes for liquid crystals:

[0004] 1. Polarization Mode

[0005] Ferroelectric Liquid Crystal (FLC);

[0006] Vertical Aligned (VA);

[0007] Hybrid Aligned Nematic (HAN);

[0008] Twisted Nematic (TN);

[0009] Electrically Controlled Birefringence (ECB);

[0010] Mixed-mode Twisted Nematic (MTN);

[0011] Self-Compensated Twisted Nematic (SCTN);

[0012] Reflected Twisted Nematic (RTN); and

[0013] Hybrid Field-Effect (HFE)

[0014] 2. Dispersion Mode

[0015] Polymer Dispersed Liquid Crystal (PDLC)

[0016] 3. Diffraction Mode

[0017] Zero Field Diffraction (ZFD)

[0018] Liquid crystals used in high picture-qualify systems are VA, MTN,etc. Particularly, VA is used for obtaining high contrast ratio.

[0019] Active-matrix displays have multiple pixels formed with a liquidcrystal filled between an active-matrix substrate and another substrate.A signal supplied to each pixel is stored in a storage capacitorprovided for the pixel, to drive the liquid crystal.

[0020]FIG. 1 shows a schematic block diagram of a typical active-matrixdisplay driven by an analog signal.

[0021] In FIG. 1, column-signal electrodes D1, D2, D3, . . . , and Diare aligned on an active-matrix substrate 2. Also aligned on thesubstrate 2 are row-scanning electrodes G1, G2, G3, . . . , and Gj,intersecting with the column-signal electrodes.

[0022] Provided at the intersection of each column-signal electrodeD(D1, D2, D3, . . . , and Di) and each row-scanning electrode G (G1, G2,G3, . . . , and Gj) is a pixel Px having a pixel-switching transistorTr, a storage capacitor Cs, and a liquid crystal 1C, as shown in FIG. 2.

[0023] A column-signal-electrode driver 100 is equipped with ahorizontal shift register 101 and several analog switches S1, S2, S3, .. . , and Si.

[0024] Input terminals of the analog switches S1, S2, S3, and Si areconnected to a display-signal supply line L through which a displaysignal VIDEO is supplied. Output terminals of the switches S1, S2, S3, .. . , and Si are connected to the column-signal electrodes D1, D2, D3, .. . , and Di, respectively. A control terminal of each switch S isconnected to the corresponding output of the horizontal shift register101.

[0025] The horizontal shift register 101 is driven by a horizontal startsignal HST and a horizontal clock signal HCK, to output pulses. Thesignals HST and HCK are supplied from a drive timing pulse generator(not shown).

[0026] The pulses output from the are horizontal shift register 101 aresupplied to the analog switches S1, S2, S3, . . . , and Si, tosequentially turn on these switches. The turn-on switches allow thedisplay signal VIDEO for one horizontal period to be sequentiallysupplied to the column-signal electrodes D1, D2, D3, . . . , and Di.

[0027] A row-scanning-electrode driver 102 is equipped with a verticalshift register having several register stages corresponding to thenumber of rows to be displayed.

[0028] The vertical shift register is driven by a vertical start signalVST and a vertical shift clock signal VCK synchronizing with onehorizontal period, to output scanning pulses. The signals VST and VCKare supplied from a drive timing pulse generator (not shown).

[0029] The scanning pulses output from the vertical shift register aresequentially supplied to the row-scanning electrodes G1, G2, G3, . . . ,and Gj per horizontal period (per row).

[0030] A vertical period of the display signal VIDEO is synchronizedwith the vertical start signal VST.

[0031] The scanning pulses turn on, sequentially per row, thepixel-switching transistors Tr connected to the row-scanning electrodesG1, G2, G3, . . . , and Gj.

[0032] Each turned-on pixel-switching transistor Tr in FIG. 2 allows thedisplay signal VIDEO, supplied to the corresponding column-signalelectrode D, to be stored as charge information in the storage capacitorCs of the corresponding pixel Px.

[0033] The stored charge information is supplied to the liquid crystal1C via a display-pixel electrode 20 for light modulation. The lightmodulation provides images to be displayed corresponding to the displaysignal VIDEO.

[0034] This type of active-matrix display, however, has the followingdisadvantages.

[0035] In FIG. 2, the scanning pulse is supplied to the gate of thepixel-switching transistor Tr to turn on the transistor to store thecharge information (display signal VIDEO) in the storage capacitor Cs.

[0036] At the moment of supplying the scanning pulse (gate voltage Vg)to the gate of the pixel-switching transistor Tr, a voltage (drainvoltage Vd) appearing at the drain of the transistor Tr rapidly variesas shown in FIG. 3 (field-through voltage Vft) due to field throughcaused by a gate-to-drain floating capacitance CGD, with respect to avoltage Vf of an electrode 21 facing the display-pixel electrode 20 inFIG. 2.

[0037] Subsequent no supply of the scanning pulse to the gate of thepixel-switching transistor Tr varies the drain voltage and then keepsthe varied drain voltage, as shown in FIG. 3.

[0038] The voltage varying as shown in FIG. 3 is then supplied to theliquid crystal LC, as D.C. (direct current) components, which causes lowphoto response and image persistence (or burn-in), etc, thus resultingin short life for the liquid-crystal panel.

[0039] In addition, this type of active-matrix display is prone togeneration of noises on the display signal VIDEO and effects of pseudodisplay signals, although provides enhanced gradation with voltagessupplied to the liquid crystal constant for one-field period but varyingin accordance with the level of the display signal VIDEO.

[0040] A method of driving a liquid crystal with pulses is disclosed inJapanese Unexamined Patent Publication No. 2001-166749, to solve theproblems discussed above.

[0041] In this method, pixels are turned on or off in accordance withthe values of bits of gradation data indicating gradation on the pixelsfor a period corresponding to weighting to the bits in each of subfieldsof one field.

[0042] Pulse-width modulation is performed in accordance with the valueof each bit of the gradation data for a bit-turn-on (-off) period in onefield, thus controlling the root mean square value of a voltage suppliedto the liquid crystal for gradation control.

[0043] Bit-turn-on (-off) signals in each subfield are bit data of lowor high level, thus not requiring analog processing circuitry, such as,a D/A converter and an operational amplifier.

[0044] Therefore, images displayed in this method are free from problemscaused by instability of circuit property and wiring resistance, etc.,which may otherwise occur when analog processing circuitry is employed.

[0045] This method is advantageous in pixel turn-on (-off) controlperformed per subfield.

[0046] Nevertheless, on- (-off) control to a drive voltage that consistsof one type of pulse to the liquid crystal could cause input/outputgradation differences based on the liquid-crystal responsecharacteristics.

[0047] In addition, this method is prone to change in drivecharacteristics among RGB liquid-crystal panels and also change in gammacharacteristics due to change in liquid-crystal response characteristicscaused by temperature change, thus disadvantageous in colorreproduction.

[0048] Pulse-driven adjustments to the drive characteristics among RGBliquid-crystal panels is, for example, disclosed in Japanese UnexaminedPatent Publication No. 6-138434 (1994).

[0049] A liquid-crystal projector disclosed here is equipped withliquid-crystal panels for R, G and B colors and a driver for controllinga pulse width of a drive pulse signal for driving each of the RGBliquid-crystal panels in accordance with input color signals to bedisplayed, for adjustments to the drive characteristics different amongthe R, G and B colors.

[0050] The drive-pulse width is varied per RGB liquid-crystal panelbased on difference in intensity curves among the panels.

[0051] It is well known that chromaticity (x, y) from white to black(dark scale) is constant at (0. 3, 0. 3) while gradation is being variedfrom white to black with projection of colors with combination of lightbeams from three liquid-crystal panels, in full-color displaying.

[0052] Difference in RGB liquid-crystal panel characteristics could,however, cause displaying of yellow for bright white or purple for darkblack.

[0053] The drive-characteristics adjustments disclosed in JapaneseUnexamined Patent Publication No. 6-138434 (1994) Achieves enhanced grayscale with pulse-width adjustments to drive voltage per RGB panel toadjust drive characteristics.

[0054] This technique is advantageous to fewer pixels and also lessgradation where as disadvantageous to high-density full-color displayingdue to circuit complexity. It is also disadvantageous in easily causingvariation in gradation, or difficulty in fine gradation adjustments dueto non-linear liquid-crystal driving. Moreover, no anti-high-temperatureresponse-speed measurements are disclosed for projectors that sufferfrom temperature rise.

[0055] Disclosed, for example, in Japanese Unexamined Patent PublicationNo. 2001-290174 is adjustments to drive parameters against change inliquid-crystal characteristics due to temperature change.

[0056] In detail, disclosed here is adjustments to input voltages inaccordance with liquid-crystal characteristics per temperature (gammacorrection) to achieve fined is playing against temperature change forhigh response-speed smectic liquid crystals suffering from change involtage-to-transmissivity characteristics due to temperature change.

[0057] Gamma correction is achieved with reconversion of 8-bit digitalsignal to 8-bit digital signal, adjustments to digital-to-analogconversion characteristics to match voltage-to-transmissivitycharacteristics and conversion of 8-bit digital signal to 10-bit digitalsignal.

[0058] The technique disclosed in this publication is advantageous inaccurate gamma correction with a digital gamma-correction circuit thatdigitally handles input digital data.

[0059] Nevertheless, the output of the digital gamma-correction circuitconverted into an analog signal by D/A conversion is prone to noises indriving a liquid crystal.

[0060] Moreover, the technique disclosed in this publication requireshigh costs on temperature sensors attached to liquid-crystal cells forgamma correction. In addition, temperature distribution over theliquid-crystal cells causes difficulty in accurate gamma correction.

SUMMARY OF THE INVENTION

[0061] A purpose of the present invention is to provide a method ofdigitally driving a liquid-crystal display with high colorreproducibility and fine gradation against temperature change.

[0062] The present invention provides a method of driving aliquid-crystal display comprising the steps of: driving one frame of aninput signal into a plurality of subframe; applying a first pulse signalto a liquid crystal irrespective of a level of the input video signal, apulse width of the first pulse signal corresponding to a duration ofeach subframe, application of the first pulse signal only not drivingthe liquid crystal; and applying a second pulse signal to the liquidcrystal in accordance with the level of the input video signal so thatsecond pulses of the second pulse signal are superimposed on the firstpulses at the same polarity, to perform pulse-width modulation to theliquid crystal, wherein an average duration P of the subframes and aresponse time L obtained by adding a rise time and a fall time of theliquid crystal meet the following requirements: P<L and P≦0.15×L.

BRIEF DESCRIPTION OF DRAWINGS

[0063]FIG. 1 shows a schematic block diagram of a typical active-matrixdisplay driven by an analog signal;

[0064]FIG. 2 shows a circuit provided for each pixel in theactive-matrix display shown in FIG. 1;

[0065]FIG. 3 illustrates a field-through voltage due to field throughcaused by a gate-to-drain floating capacitance;

[0066]FIG. 4 shows a block diagram of a PWM-based liquid crystal displayaccording to the preset invention;

[0067]FIG. 5 shows a pulse driver provided for each pixel in thePWM-based liquid crystal display shown in FIG. 4;

[0068]FIG. 6 shows timing charts indicating an operation of the circuitshown in FIG. 5;

[0069]FIG. 7 illustrates exemplary pulse widths of a first and a secondpulse signal;

[0070]FIG. 8 shows a graph indicating brightness (reflectivity) to theroot mean square value of pulse signals;

[0071]FIG. 9 illustrates other exemplary pulse widths of a first and asecond pulse signal;

[0072]FIG. 10 shows a graph indicating change in output light to voltageof a pulse drive signal within one frame divided into 22 subframes;

[0073]FIG. 11 illustrates a response speed of a liquid crystal;

[0074]FIG. 12 shows drive characteristics (output light intensity vs.input signal) of a reflective liquid crystal display; and

[0075]FIG. 13 shows change in drive characteristics of a liquid crystaldisplay against change in response time of a liquid crystal.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

[0076] An embodiment according to the present invention will bedisclosed with reference to the attached drawings.

[0077]FIG. 4 shows a PWM-based liquid crystal display according to thepreset invention, elements thereof the same as or analogous to thoseshown in FIG. 1 being given the same reference numerals and signs, hencenot disclosed in detail.

[0078] An input vide signal VIDEO is supplied to an A/D converter 12 tobe converted into a digital signal. The digital signal is supplied tocolumn-electrodes D1 to Di via analog switches S1 to Si, respectively.

[0079] The digital signal supplied to each of the column-electrodes D1to Di is stored in an SRAM (static RAM) 4 for each pixel shown in FIG. 5(a pulse driver).

[0080] A pulse-width modulator (PWM) 13 shown in FIG. 4 is connected toa buffer circuit 6 in FIG. 5. It sets a period of time for applying thesignal stored in the SRAM 4 to a liquid crystal LC that is connected toa common electrode CE.

[0081] The digital vide signal VIDEO is further inverted by an inverter14 and supplied to column-electrodes *D1 to *Di via switches S1′ to Si′,respectively. The switches are turned on or off in synchronism with theanalog switches S1 to Si.

[0082] The pulse driver shown in FIG. 5 for PWM-based digitalliquid-crystal driving withstands effects of floating capacity betweenthe electrodes because of each fixed electrode potential, thus far lessprone to several problems on displayed images compared to analogliquid-crystal driving.

[0083] The pulse driver shown in FIG. 5 is equipped with the SRAM 4 forstoring the digital input video signal VIDEO and the buffer circuit 6for transferring the stored signal to each pixel electrode 20.

[0084] The SRAM 4 has a flip-flop of transistors Tr1 to Tr4. It isconnected to the column-signal electrode D for the stored video signalVIDEO and the other column-signal electrode *D for a video signal *VIDEO(the inversion of the stored signal VIDEO) via switching transistors S20and S10, respectively.

[0085] The digital input video signal VIDEO is temporarily stored in theSRAM 4 when a pulse is supplied to the gates of the switchingtransistors S20 and S10 via the row-scanning electrode G, in synchronismwith the video signals VIDEO and *VIDEO flowing through thecolumn-signal electrodes D and *D, respectively.

[0086] An external signal supplied to the buffer circuit 6 turns on aswitch (not shown) of the circuit 6 to allow the video signal VIDEOstored in the SRAM 4 to be supplied to the liquid crystal LC to driveliquid-crystal molecules.

[0087] In timing charts shown in FIG. 6 for the pulse driver in FIG. 5,waveforms A and B indicate potentials at nodes A and B, respectively,shown in FIG. 5.

[0088] The liquid crystal 1C is a negative dielectric anisotropic liquidcrystal exhibiting orientation almost perpendicular to an active-matrixsubstrate while no voltage is being applied.

[0089] Disclosed next in detail is a method of driving the liquidcrystal display configured as above, according to the present invention.

[0090] The features of the driving method according to the presentinvention are as follows:

[0091] Generated first is a first pulse signal, pulses thereof beinginverted in one frame divided into, for example, 22 subframes. Generatednext is a second pulse signal, pulses thereof being superimposed on thepulses of the first pulse signal at the same polarity.

[0092] The first pulse signal is adjusted so that integration of thepulses is almost zero in one frame. The level of the first pulse signalis further adjusted so that application of this pulse signal only doesnot drive the liquid crystal.

[0093] Pulse-width modulation is performed when the second pulse signalis superimposed on the first pulse signal.

[0094] An average duration P of the subframes and a response time L(addition of a rise time and a fall time) of the liquid crystal meet thefollowing requirements:

[0095] P<L and

[0096] P<0.15×L

[0097] Pulse-width modulation is performed at the PWM 13 as follows:

[0098] A general driving method with a first pulse signal and a secondpulse signal superimposed thereon causes output-light stepped portionsin input signal-to-output light characteristics.

[0099] To avoid such output-light stepped portions, in the presentinvention, the input video signal is supplied to the liquid crystal as acombination of the first pulse signal and the second pulse signal.

[0100] The first pulse signal is always applied to the liquid crystalirrespective of the level of the input video signal whereas the secondpulse signal is applied to the liquid crystal in accordance with thelevel of the input video signal level.

[0101] The first and second pulse signals P_(1st) and P_(2nd) areillustrated in FIG. 7. A pulse width L1 of the first pulse signalcorresponds to the length or duration of each subframe within one frameF. The second pulse signal has a pulse width 12.

[0102] Shown in FIG. 8 is graph showing brightness (reflectivity) to theroot mean square value of the pulse signals.

[0103] The amplitude of the first pulse signal is set at a thresholdvoltage P0 just before the liquid crystal is subjected to modulation.The amplitude of the second pulse signal is set at a voltage P2 passinga peak voltage P1 at which the liquid crystal produces the maximumbrightness (reflectivity), or between the peak voltage P1 and thethreshold voltage P0.

[0104] As shown in FIG. 7, the pulses of the first pulse signal P_(1st)are being inverted in one frame and those of the second pulse signalP_(2nd) is superimposed thereon at the same polarity.

[0105] The first pulse signal is adjusted so that integration of thepulses is almost zero in one frame. The level of the first pulse signalis further adjusted so that application of this pulse signal only doesnot drive the liquid crystal.

[0106] In other words, the second pulse signal is subjected topulse-width modulation. In detail, pulse-width modulation is performedsuch that pulses of all subframes in one frame are set at a positive ora negative polarity in the maximum modulation, integration of thepositive and negative pulses in one frame being zero, so as not to applya D.C. voltage to the liquid crystal. The second pulse signal isinverted per frame.

[0107] The liquid crystal LC (FIG. 5) in this invention is a negativedielectric anisotropic liquid crystal exhibiting orientation almostperpendicular to an active-matrix substrate while no voltage is applied.

[0108] Using such type of liquid crystal in this invention offers a veryclear black on screen while no voltage is applied (normally black).Pulse-width modulation (PWM 13 in FIG. 4) to such type of liquid crystalin this invention requires only a small driving voltage to the liquidcrystal thanks to a quick rise time but a long fall time, which is not aD.C. voltage within one frame and also over frames, thus achievinghighly reliable displaying of high quality images.

[0109] Illustrated in FIG. 9 is an alternative pulse wave forms for oneframe F available in this invention. A first pulse signal P_(1st) and asecond pulse signal P_(2nd) superimposed thereon have the same pulsewidth within a subfield S.

[0110] A ratio of the width of the second pulse signal to that of thefirst pulse signal is larger the better, close to 1, as much aspossible, and preferable always constant.

[0111] Illustrated in FIG. 10 is graph indicating change in output lightto voltage of a pulse drive signal within one frame divided into 22subframes Sf.

[0112] The embodiment offers normally black on screen using a liquidcrystal exhibiting orientation almost perpendicular to an active-matrixsubstrate while no voltage is applied, causing no twist toliquid-crystal molecules with almost no portions to loose brightness,thus achieving bright displaying.

[0113] In addition, the liquid crystal exhibits orientation almostperpendicular to an active-matrix substrate against a threshold voltage,thus offering a very clear black on screen, while no voltage is applied,for high contrast.

[0114] Moreover, pulse-width adjustments to each of the first and thesecond pulse signals achieves enhanced gradation with almost nostep-like variation in output light which may otherwise occur in bitchange.

[0115] As already disclosed, one feature of the present invention isthat the average duration P of the subframes, which is an average pulsewidth of the first pulse signal, and the response time L (addition of arise time and a fall time) of the liquid crystal meet the followingrequirements:

[0116] P<L and

[0117] P<0.15×L

[0118] The average duration P is an average of durations of subframeswithin one frame F, the duration L1 of each subframe (width of eachpulse in the first pulse signal) being illustrated in FIG. 7.

[0119] Response speed of the liquid crystal LC is illustrated in FIG. 11that shows an input pulse to the liquid crystal LC and output lighttherefrom.

[0120] A rise time Tr is a period for the output light from the liquidcrystal 1C increasing from 10% to 90% after the input pulse rises.

[0121] A fall time Td is a period for the output light from the liquidcrystal 1C decreasing from 90% to 10% after the input pulse falls.

[0122] The response time L discussed above is thus given by L=Tr+Td,which offers high color reproducibility with almost no effects of changein viscosity of the liquid crystal LC due to temperature change, etc.

[0123] The high color reproducibility in this invention will bediscussed for 512 gradations (29) with a 9-bit video signal.

[0124] Nine-bit video signals basically require 9 subframes per frame,however, have 22 subfarames per frame, for example, as illustrated inFIG. 10, to avoid generation of pseudo-edges in moving pictures.

[0125]FIG. 10 shows a waveform of the 9-bit video signal for one frame,divided into 22 subfarames per field.

[0126] One-frame duration at 16.67 msec gives about 0.76 msec for theaverage subframe duration P.

[0127] The liquid crystal LC may be of any type among VA, HAN, TN, ECB,MTN, SCTN, RTN and HFE modes.

[0128] Used in the following disclosure is the AV mode in which negativedielectric anisotropic nematic liquid crystals are vertically oriented.

[0129] A pretilt angle and retardation of the liquid crystal LC are 85°and 270 nm, respectively, in the graph shown in FIG. 8. Retardation isgiven by multiplication of birefringence and cell thickness of theliquid crystal LC.

[0130] Plotted on FIG. 8 are the root mean square values of a drivevoltage (rectangular waveform) on the axis of abscissas (normalizedscale) and modulated brightness (reflectivity) from the liquid crystalLC on the axis of ordinate (normalized scale).

[0131] Disclosed below is driving the liquid crystal 1C with pulse drivevoltages with respect to FIG. 8 in actual application.

[0132] The amplitude (voltage) of the first pulse signal is set at thepoint P0 to gain a contrast ratio of 300:1 for the liquid crystal 1C.The amplitude (voltage) of the second pulse signal is set at the pointP2 passing the peak voltage P1, to gain enough output light.

[0133] These voltage settings are adjusted in accordance with RGBcolors.

[0134] The first pulse voltages may be 1.65 V, 1.6 V and 1.4 V for R, Gand B colors, respectively, and the second pulse voltages may be 5.2volts, 4.7 volts and 4.4 volts for the R, G and B colors, respectively,when the center wavelengths are 610 nm, 550 nm and 450 nm for the R, Gand B colors, respectively.

[0135]FIG. 12 shows drive characteristics (output light intensity vs.input signal) of a reflective liquid crystal display driven by the firstand second pulse signals adjusted as explained above. The reflectiveliquid crystal display exhibits different drive characteristics over theRGB colors.

[0136] Gamma correction different among the RGB colors to these drivecharacteristics to gain the same drive characteristics can basicallyprevent color imbalance in accordance with input signals, with a look-uptable of gamma-correction coefficients.

[0137] Nevertheless, such a look-up table is useless in actualapplications due to the following reasons:

[0138] The liquid-crystal response speed depends on several factors inliquid crystal, such as, material, orientation, cell thickness, anddrive voltage.

[0139] Particularly, the liquid-crystal response speed varies inprojection liquid-crystal displays due to temperature rise in aliquid-crystal panel when a liquid crystal is exposed to intense lightbeams.

[0140] Temperature rise in a liquid-crystal panel is inevitable forprojection liquid-crystal displays although drive voltages arecontrollable and the material, orientation, and cell thickness can betreated as constant factors once the liquid-crystal panel is produced.

[0141] Moreover, pulse-width modulation to drive the liquid-crystalpanel causes change in input/output characteristics and also gammacharacteristics, which depends on the pulse rise and fall time. Theloop-up table explained above cannot offer accurate input/outputcharacteristics.

[0142] As already disclosed, one of the features of the presentinvention to solve such problems is as follows:

[0143] An average pulse width P for the first pulse signal (averageduration of subframes) is given by P<0.15×L in which L is a responsetime L (addition of a rise time and a fall time) of the liquid crystalmeet the following requirements:

[0144] P<L and

[0145] P≦0.15×L

[0146] Shown in FIG. 13 is change in drive characteristics of a liquidcrystal display against change in response time of the liquid crystal.

[0147] The response time L (rise time Tr+fall time Td shown in FIG. 11)was varied in the range from 3.2 to 31.6 msec.

[0148]FIG. 13 shows that the liquid crystal display has the followingtendency:

[0149] Increase output light in a dark section (the left side of FIG.13) when the response time L becomes high (or small) because the liquidcrystal exhibits high response in accordance with input pulses; incontrast, decrease output light in a bright section (the right side ofFIG. 13) even though the response time L becomes high (or small).

[0150] Also shown in FIG. 13 is the opposite tendency as follows:

[0151] Decrease output light in the dark section (the left side of FIG.13) when the response time L becomes low (or large); in contrast,increase output light in the bright section (the right side of FIG. 13)even though the response time L becomes low (or large).

[0152] Moreover, FIG. 13 teaches that the output light does not vary atthe center section X1 of drive characteristic curves with almost noeffects of change in the response time L.

[0153] The response time L for acceptable change in drivecharacteristics is 5.0 msec (critical point) or larger, preferably, 15.9 msec (critical point) or larger but smaller than 30 msec (which willbe discussed later).

[0154] When driven by pulse-width modulation, a liquid crystal exhibitsexcellent drive characteristics as the width of a drive pulse signalbecomes smaller in relation to the response time (P≦a·L in which a is acoefficient) while the signal is being inverted per subframe.

[0155] Substitution of 0.76 msec (discussed with respect to FIG. 10) and5.0 msec for P and L, respectively, in P≦a·L gives a≦0.15.

[0156] It is therefore understood that meeting the requirement P≦0.15×Loffers high color reproducibility and also high moving-picturecharacteristics with almost no effects of temperature change in liquidcrystals.

[0157] In detail, smaller drive pulse width with the requirement P≦0.15×L offers constant gamma characteristics with almost no effects ofviscosity of liquid crystals, thus achieving enhanced gradation with notemperature sensors.

[0158] The liquid-crystal response time L is preferably shorter than 30msec, which otherwise causes image persistence, in relation to themoving-picture characteristics.

[0159] Substitution of 30 msec for L in P≦0.15×L, or 0.15×30, gives 4.5msec, thus meeting the requirement P≦0.15×L<4.5, enhancing the colorreproducibility and moving-picture characteristics.

[0160] A larger number of subframes require a higher transfer rate fortransferring input signals. High transfer rate may be achieved withparallel signal input.

[0161] A monocrystal silicon active matrix substrate allows high densesignal output therefrom with easily assembled parallel signal outputcircuitry, and further allows drive circuitry to be directly built in aliquid-crystal display.

[0162] As disclosed in detail, according to the present invention, aliquid-crystal display is driven by pulse-width modulation as follows:

[0163] One frame of an input video signal is divided into a plurality ofsubframes.

[0164] A first pulse signal is applied to a liquid crystal irrespectiveof the level of the input video signal. A pulse width of the first pulsesignal corresponds to the duration of each subframe. Application of thefirst pulse signal only does not drive the liquid crystal.

[0165] A second pulse signal is applied to the liquid crystal inaccordance with the level of the input video signal so that secondpulses of the second pulse signal are superimposed on the first pulsesat the same polarity, to perform pulse-width modulation to the liquidcrystal.

[0166] An average duration P of the subframes and a response time Lobtained by adding a rise time and a fall time of the liquid crystalmeet the requirements P<L and P≦0.15×L.

[0167] Accordingly, the method of driving a liquid-crystal display witha pulse (digital) drive signal according to the invention achievesdisplaying images with high color reproducibility and gradation, withsimple external circuitry, even when suffering temperature change ornoises such as crosstalk.

What is claimed is:
 1. A method of driving a liquid-crystal displaycomprising the steps of: dividing one frame of an input signal into aplurality of subframes; applying a first pulse signal to a liquidcrystal irrespective of a level of the input video signal, a pulse widthof the first pulse signal corresponding to a duration of each subframe,application of the first pulse signal only not driving the liquidcrystal; and applying a second pulse signal to the liquid crystal inaccordance with the level of the input video signal so that secondpulses of the second pulse signal are superimposed on the first pulsesat the same polarity, to perform pulse-width modulation to the liquidcrystal, wherein an average duration P of the subframes and a responsetime L obtained by adding a rise time and a fall time of the liquidcrystal meet the following requirements: P<L and P≦0.15×L.
 2. Thedriving method according to claim 1 further comprising the step ofadjusting the first pulse signal so that integration of the first pulsesis almost zero in one frame.
 3. The driving method according to claim 1,wherein the second-pulse-signal application step comprising the step ofperforming the pulse-width modulation such that pulses of all of thesubframes in one frame are set at a positive or a negative polarity inmaximum modulation so that integration of the positive and negativepulses in one frame is zero.
 4. The driving method according to claim 1further comprising the step of setting an amplitude of the first pulsesignal at a threshold voltage just before the liquid crystal issubjected to the pulse-width modulation and an amplitude of the secondpulse signal at a voltage between the threshold voltage and a peakvoltage at which the liquid crystal produces the maximum brightness orreflectivity.